The majority of plastic films produced are employed in some form of packaging. The present invention is primarily concerned with those films used for applications requiring a low level of oxygen in a package, though may also find other uses. Primarily this type of packaging includes comestibles such as meat, which are often packaged to have a low oxygen environment.
Limiting the exposure of oxygen-sensitive products to oxygen maintains and enhances the quality and shelf life of many products. For instance, by limiting the oxygen exposure of oxygen-sensitive food products in a packaging system, the quality of the food product can be maintained and spoilage retarded. In addition, such packaging also keeps the product in inventory longer, thereby reducing costs incurred from waste and having to restock.
In the food packaging industry, several techniques for limiting oxygen exposure have been developed. Common techniques include those where oxygen is consumed within the packaging environment by some means other than the packaged article or the packaging material (e.g., through the use of oxygen scavenging sachets), those where reduced oxygen environments are created in the package (e.g., modified atmosphere packaging (MAP) and vacuum packaging), and those where oxygen is prevented from entering the packaging environment (e.g., barrier films).
The art dealing with barrier packaging, and the low oxygen or modified packaging of products is relatively well developed. This includes the use of films and inserts containing oxygen scavenging compounds able to extract a majority of any residual oxygen after packaging occurs.
Oxygen scavenging compounds for use in plastic films are relatively well known.
For instance:
Michael Rooney, "Oxygen scavenging: a novel use of rubber photo-oxidation", Chemistry and Industry, Mar. 20, 1982, pp 197-198 describes the use of ethylenically unsaturated compounds as oxygen scavengers on exposure to light. However systems describing the use of transition metal catalysts are not described.
U.S. Pat. No. 4,908,151 to Mitsubishi, describes sachets containing unsaturated fatty acid (i.e. an ethylenically unsaturated hydrocarbon) in combination with a transition metal compound in a basic substance. However there is no description of these materials in the form of a film nor the use of photo-exposure as an initiating mechanism. The disadvantages of sachets include the need for additional packaging steps (to add the sachet to the package), the potential for contamination of the packaged article should the sachet break, and the potential of ingestion by a consumer.
Japanese patent JP5032277 to Kuwa describes the use of radical containing resin layers in packages. The invention comprises an oxidizable polymer whose oxygen scavenging abilities is photoinitiated.
New Zealand patent application NZ241802 to W. R. Grace and also NZ243077 also to W. R. Grace, claim oxygen scavenging compositions comprising ethylenically unsaturated hydrocarbons with transition metal catalysts. A wide range of ethylenically unsaturated compounds are discussed in the texts of these specifications though there is no mention of the problem to which the present invention is directed, nor the compounds and products encompassed by the present invention.
Oxygen scavenging materials also have been incorporated directly into the packaging structure. This technique (hereinafter referred to as "active oxygen barrier") can provide a uniform scavenging effect throughout the package and can provide a means of intercepting and scavenging oxygen as it passes through the walls of a package, thereby maintaining the lowest possible oxygen level throughout the package. Active oxygen barriers have been formed by incorporating inorganic powders and/or salts as part of the package. See, e.g., U.S. Pat. Nos. 5,153,038, 5,116,660, 5,143,769, and 5,089,323. However, incorporation of such powders and/or salts can degrade the transparency and mechanical properties (e.g., tear strength) of the packaging material and can complicate processing, especially where thin films are desired. Also, these compounds as well as their oxidation products can be absorbed by food in the container, which can result in the food product failing to meet governmental standards for human consumption.
EP 0 519 616 discloses an oxygen scavenging composition that includes a blend of an epoxide, a first polymeric component grafted with an unsaturated carboxylic anhydride and/or acid, a second polymeric component including OH, SH, or NHR.sup.2 groups where R.sup.2 is H, C.sub.1-C.sub.3 alkyl, or substituted C.sub.1 -C.sub.3 alkyl moiety, and a metal salt capable of catalyzing the reaction between oxygen and the second polymeric component. The first polymeric component is present in an amount sufficient to ensure that the blend is non-phase separated. A blend of polymers is utilized to obtain oxygen scavenging, and the second polymeric component is preferably a (co)polyamide such as MXD6.
Another type of active oxygen barrier is illustrated in EP-A-0 301 719, EP-A-0 380 319, PCT publication no. WO 90/00578, and PCT publication no. WO 90/00504. See also U.S. Pat. Nos. 5,021,515, 5,194,478, and 5,159,005. The disclosed oxygen scavenger includes polyamide-transition metal catalyst compositions. Through catalyzed scavenging by the polyamide, the package wall regulates the amount of oxygen reaching the interior of the package. However, the onset of useful oxygen scavenging (i.e., up to about 5.8 .times.10.sup.-5 cm.sup.3 /m.sup.2.s or 5 cm.sup.3 /m.sup.2.24 hours at ambient conditions) can take as long as 30 days to occur. Therefore, this technique is not acceptable for many applications. Further, polyamides typically are incompatible with many thermo-plastic polymers commonly used to make flexible packaging materials (e.g., ethylene/vinyl acetate copolymers, low density polyethylene, etc.) or, when used by themselves, are difficult to process and result in inappropriately stiff structures.
Oxygen scavenging compositions that include transition metal catalysts and ethylenically unsaturated hydrocarbon polymers which have an ethylenic double bond content of from 0.01 to 10 equivalents per 100 grams of polymer are disclosed in U.S. Pat. No. 5,399,289. Various conventional homopolymers, copolymers, and polymer blends are disclosed. Because these polymers are amorphous, they can be difficult to blend and process with film-forming semicrystalline polymers conventionally used to make flexible packaging materials.
The use of a transition metal and a photoinitiator to facilitate initiation of effective scavenging activity of ethylenically unsaturated compounds is taught in U.S. Pat. No. 5,211,875, which is incorporated herein by reference as if set forth in full.
PCT publication nos. WO 95/02616 and WO 96/40799 disclose a scavenger composition that includes a transition metal salt and a copolymer (of ethylene and a vinyl monomer) having ether, amino, carboxylic acid, ester, or amide functionalities pendent therefrom. Although these compositions can provide oxygen scavenging activity, the particular advantages of having ethylenic unsaturation contained within a cyclic moiety are not disclosed. Because the compositions of this invention are significantly cleaner than those described in the prior art, they do not require the use of high levels of adjuncts to absorb the undesirable byproducts. Such absorbent additives are known in the art, for example see U.S. 5,834,079 and U.S. 08/857,276. It is also well know in the art that such additives (zeolites and silicas) adversely effect the haze and clarity of packaging structures.
PCT Application WO 96/40799 describes the use of a variety of ethylenic materials with benzylic, or ether containing side chains. Some of these materials may be prepared by esterification or transesterification of a polymer melt. The use of pendent cyclic groups containing unsaturation is generally referred to, but there is only one such example, wherein Nopol, a bicyclic alcohol, is used in a transesterification reaction and oxygen absorbing films are formulated from the product. There is no reference to the benefits of cyclic compounds as described in this invention i.e., on oxidation they produce very low levels of oxidation byproducts when compared to comparable linear systems. Because of its bicyclic nature, Nopol is not expected to produce these benefits.
While the prior art compounds may effectively scavenge oxygen they introduce other problems into packaging. For instance, in summary the prior art incorporates into film structures compounds which are ethylenically unsaturated but which often cleave as a consequence of the reactions of the oxygen scavenging process. For example, films containing unsaturated compounds such as squalene or vegetable oils produce large amounts of volatile aldehydes and ketones upon oxidation. Unfortunately many of these volatile compounds are not contained within the film structure and find their way into the head space of the package. Here they can represent more of a problem than the oxygen which they have replaced and have the potential to contaminate comestible products.
This problem represents a significant problem yet has been downplayed or overlooked by the published prior art. As a consequence, those searching the prior art for a solution to this problem find no answer--the art appears to be directed primarily along a narrow track of improving on scavenging efficiencies, or physical properties of scavenging films, rather than recognizing or addressing other associated problems.
Accordingly the present invention seeks to address the problems associated with scission products of oxygen scavengers, and seeks also to provide a group of compounds and substances (as well as films and plastic materials including same) which have an advantage over the prior art in terms of reduced quantities of scission products.
Ideally, a polymeric material for use in an oxygen scavenging composition should exhibit good processing characteristics, be able to be formed into useful packaging materials or have high compatibility with those polymers commonly used to make packaging materials, and not produce byproducts which detract from the color, taste, or odor of the packaged product. It has been found that when the ethylenic unsaturation is contained within a cyclic group, substantially fewer and less byproducts are produced upon oxidation as compared to analogous non-cyclic materials. Optimally, a packaging material formed from such a composition can retain its physical properties after significant oxygen scavenging.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.